How to fix the shadow line in the middle of the face?
This is probably due to how Blender is currently rendering your mesh's normals. A normal vector is a vector which is orthogonal to a plane. In this case, the faces of your mesh.
The configuration of your normals tells Blender how light should interact with the faces of your mesh. Is your face a flat surface, or is it meant to represent a rounded side of a cylinder or sphere? Is the face facing away from you, or toward you?
The first thing to check is whether or not you have any normals pointing the wrong way. In Edit mode, selecting everything with A, and press SHIFT+N to recalculate normals.
On more complex, or non-manifold geometry, this doesn't always work as desired. You can manually flip the normals of particular faces by selecting them and hitting ALT+N, then F.
Correcting Flipped Normals:

The next possibility is that Blender simply isn't displaying the normals in the way you want. Blender can display the accurate normal of each individual face (flat shading), or it can average these normals across faces (smooth shading). Finally, it can combine these two effects based on a cut-off angle.

You can switch between flat and smooth shading from the Object menu while in Object mode by selecting either Shade Flat or Shade Smooth. Auto Smooth is selected from the Object Data Properties tab of the Properties shelf.